1. Field of the Invention
The present invention relates to a method and apparatus for drilling a borehole into a subsea abnormal pore pressure environment. In particular, the present invention discloses a method and apparatus for drilling a borehole through subsea geological formations while maintaining the fluid pressure inside of the borehole equal to or greater than the pore pressure in the surrounding geological formations using a fluid, that is of insufficient density to generate a borehole pressure greater than the surrounding geological formations pore pressures without borehole fluid pressurization.
2. Description of the Related Art
Drilling fluid using additives to provide an elevated density is generally used to control and stabilize a borehole during the drilling process in an abnormal pore pressure environments. In particular, additives, such as barite and clay, are added to the drilling fluid to increase its density and viscosity. When drilling in deep water, a riser has been used to allow the drilling fluid to be circulated back to a floating drilling vessel. The riser must be large enough in diameter to accommodate the largest bit and casing that will be used in drilling the borehole. As drilling is extended into deeper water, the size of the riser becomes difficult to handle with current floating drilling vessels. The use of larger floating vessels to handle larger risers would greatly increase the daily rentals of these vessels (some now approaching $200,000/day), which would make production economical only for high rate producing wells. Therefore, this economic constraint could limit deep water oil and gas developments.
To reduce the cost related to conventional risers, the initial shallow part of the well is often drilled and cased with a conductor tubular or casing without using a riser. In this shallow drilling and casing, an economical single-pass drilling fluid, such as sea water, is used. Therefore, since sea water is used as the drilling fluid it can be discharged into the sea, without having to be pumped back up to the floating vessel.
Sea water may be used as a conventional drilling fluid in a normal pore pressure environment because sea water has sufficient density to control and stabilize a borehole through geological formations in a normal pressured environment throughout the drilling process. In other words, the shallow part of the well can be drilled with conventional techniques into the normal pore pressure environment without a riser because the borehole pressure is controlled by the hydrostatic pressure of the sea water. Later in the well drilling process when a subsea borehole is drilled into an abnormal pore pressure environment, additives to the drilling fluid are commonly used. It has been found that abnormally pressured aquifers are often encountered at very shallow depths in many deepwater geological environments. Most past attempts to drill through these shallow abnormally pressured aquifers with sea water have been unsuccessful. Conventional drilling fluids with additives are not commonly used as a single pass drilling fluid, as is sea water, because the large amount of additives which would have to be discharged during continuous drilling operations would make the process uneconomical.
As best shown in FIG. 1, a flow from a shallow abnormally pressured aquifer 10 washes out sand and soils, such as from the formation and over-lying sediments collapse. The collapse of the sediments can create a flow path 12 in the sea floor SF. This erosion and destabilization of the sea floor can undermine expensive subsea wells and make the site unsuitable for anchoring tension leg platforms and spar platforms. The destabilization of the sea floor can also collapse the casings of previously drilled wells at the site. Thus, this erosion could force the abandonment of locations where hundreds of millions of dollars have been invested. Current practice for minimizing this risk is to batch drill the shallow section of all the wells in a location to a depth below the shallow abnormally pressured aquifers. If the location is lost due to an uncontrolled water flow from the aquifer while following this current practice, the minimum possible investment is lost.
Turning now to FIG. 2, conventional drilling technology using a riser 34 for drilling a subsea borehole is shown. The depth 14 to the top of the abnormal pore pressure environment, in this example, is 1500 feet when referenced to the mudline 16 of the sea floor SF or a depth 18 of 4,780 feet when referenced to the kelly bushing 20 on the rig floor of the floating vessel 22.
An abnormal pore pressure environment is an environment where the hydrostatic pressure of sea water will not control the pore pressure. In other words, an abnormal pore pressure environment is an environment where the hydrostatic pressure generated by a column of sea water is less than the pore fluid pressure of the geological formations surrounding the borehole.
Therefore, the example well in FIG. 2 can be safely drilled using sea water as the drilling fluid to a depth of 1500 feet, since the pore pressure 24A in the normal pore pressure environment is equal to the hydrostatic pressure of sea water (8.6 ppg) 26 down to this depth. In other words, the increase in the pore pressure gradient at 1500 feet, as seen by the slope change at 28, indicates that the maximum casing depth for conductor or the first casing is 1500 feet. Since the maximum setting depth of the first casing in the example well is a function of the pore pressure 24A and the fracture pressure 30, the drilling and casing can be accomplished without a riser using sea water. Therefore, as discussed above, the first portion (the "normal pressure" portion of the well) of the borehole above 1500 feet in this example well is drilled and cased without a riser.
Since the first portion of the borehole (above 1500 feet) can be drilled without a riser, the diameter of the first casing 32 is not limited by the riser 34. However, in this example, the riser 34 (and a drilling fluid having additives) is required when using conventional drilling technology to drill depths greater than 1500 feet. Because casing 36 must pass through the conventional 18 inch diameter clearance blowout preventer (BOP) stack 37 and 18 inch diameter riser 34, selecting a casing size for the first casing 32 which is much larger than the riser 34 and BOP stack 37 clearance is of no benefit. Presently, most floating vessels do not handle risers bigger than 19 inches in diameter. As a result, risers greater than 19 inches in diameter are not used. The cost of upgrading the floating vessel for a larger riser diameter is quite high due to the increase load which would be imposed on the vessel when handling the larger, heavier risers. As a result, the second casing 36 run into the conventional-drilled well, such as shown in FIG. 2, is limited by the riser 34 (18 inches in diameter) and must have clearance to pass through a 18 inch diameter. Therefore, in the example of FIG. 2, selecting the size of the first casing 32 to be as close as possible to the riser 34 diameter is the most cost effective. Thus, the selected size of first and second casings run into the conventionally-drilled example well are 20 inches and 16 inches, respectively. In order to maintain the borehole fluid pressure for the second casing between the pore pressure 24B and the fracture pressure 30, the borehole for the second casing can be drilled to approximately 2,560 feet, as shown graphically in FIG. 2. The line 38 (indicating use of a fluid with additives-a 10.1 ppg mud) is maintained between the pore pressure line 24B and the fracture line 30 with a 200 psi safety margin while being drilled through the shallow aquifer 10 to 2,560 feet.
U.S. Pat. No. 4,813,495 proposes an alternative to the conventional drilling method and apparatus of FIG. 2 by using a subsea rotating control head in conjunction with a subsea pump that returns the drilling fluid to a drilling vessel. Since the drilling fluid is returned to the drilling vessel, a fluid with additives may economically be used for continuous drilling operations. ('495 patent, col. 6, ln. 15 to col. 7, ln. 24) Therefore, the '495 patent moves the base line for measuring pressure gradient from the sea surface to the mudline of the sea floor ('495 patent, col. 1, lns. 31-34). This change in positioning of the base line removes the weight of the drilling fluid or hydrostatic pressure contained in a conventional riser from the formation. This objective is achieved by taking the fluid or mud returns at-the mudline and pumping them to the surface rather than requiring the mud returns to be forced upward through the riser by the downward pressure of the mud column ('495 patent, col. 1, lns. 35-40). Therefore, the '495 patent, while acknowledging the economic and environmental concerns of dumping, proposes the use of drilling fluids with additives that are only dumped to the sea floor in an emergency. ('495 patent, col. 3, lns. 30-35)